1. Field of the Invention
The present invention relates to a magnetic random access memory having memory cells, each using a magneto resistive element that stores data by means of a magneto resistive effect.
2. Description of the Related Art
“Magnetic random access memory” (which will be referred to as MRAM) is a generic name of solid memories that can rewrite, hold, and read record information, as the need arises, by utilizing the magnetization direction of a ferromagnetic body used as an information recording carrier.
In general, each of the memory cells of the MRAM has a structure in which a plurality of ferromagnetic bodies are stacked one on the other. Information recording is performed by assigning units of binary information “1” and “0” respectively to parallel and anti-parallel states, i.e., the relative positions in magnetization, of the plurality of ferromagnetic bodies forming each memory cell. When record information is written, the magnetization direction of a ferromagnetic body of each cell is inverted by a magnetic field generated by electric currents fed through write lines, which are disposed in a criss-cross fashion. The MRAM is a nonvolatile memory, which, in principle, has zero power consumption during record holding, and record holding is maintained even after power off. Record information is read by utilizing the so-called magneto resistive effect, in which the electric resistance of each memory cell varies in accordance with the angle between the magnetization direction of a ferromagnetic body in each memory cell and the sense current, or angle between the magnetization directions of a plurality of ferromagnetic layers.
The MRAM has many advantages in function, as shown in the following (1) to (3), as compared to conventional semiconductor memories using a dielectric body. (1) It is completely nonvolatile, and allows the number of rewriting operations to be more than 1015. (2) It allows nondestructive reading, and requires no refreshing operation, thereby shortening read cycles. (3) As compared to memory cells of the charge accumulation type, it has a high radiation-tolerance. The MRAM may have an integration degree per unit area, and write and read times, almost the same as those of the DRAM. Accordingly, it is expected that the DRAM will be applied to external recording devices for portable equipment, hybrid LSIs, and primary storage for personal computers, making the most of the specific feature of nonvolatility.
At present, feasibility studies are being carried out regarding practical use of MRAMs, in which each memory cell employs, as a magneto resistive element, an MTJ (Magnetic Tunnel Junction) element that forms a ferromagnetic tunnel junction (for example, ISSCC 2000 Digest Paper TA7.2). The MTJ element is formed mainly of a three-layered film, i.e., ferromagnetic layer/insulating layer/ferromagnetic layer, in which an electric current flows by tunneling through the insulating layer. The electric resistance value of the junction varies in proportion to the cosine of the relative angles in magnetization of the two ferromagnetic metal layers. The resistance value becomes maximum when the magnetization directions are in anti-parallel with each other. This is called a TMR (tunneling magneto resistive) effect. For example, in the case of NiFe/Co/Al2O3/Co/NiFe, a magneto resistive change rate of more than 25% is observed with a low magnetic field of 50 Oe or less.
As a structure of the MTJ element, there is known a retentivity difference type, which utilizes the difference in retentivity between two ferromagnetic bodies to hold data. There is also known a so-called spin valve structure type, in which an anti-ferromagnetic body is disposed adjacent to one of two ferromagnetic bodies to fix its magnetization direction, so as to improve the magnetic field sensitivity or to reduce the electric current in writing (for example, Jpn. J. Appl. Phys., 36, L200 (1997)).
However, in order to develop MRAMs having an integration degree of Class-Gb, there are several problems left to solve. One of them is that a variation in junction resistance due to the variation in processing MTJ elements is not negligible as compared to the TMR effect, and makes reading very difficult. A reading operation of the self-reference type can be used to solve this problem, and an example of this is explained below.
First, a value of an electrical-property based on stored data in a selected memory cell at a read target address is detected and stored in a data buffer. Then, “1” data is written in the selected memory cell and read therefrom, so that a value of the electrical property based on the “1” data is detected and stored in a “1” data buffer. Then, “0” data is written in the selected memory cell and read therefrom, so that a value of the electrical property based on the “0” data is detected and stored in a “1” data buffer. Lastly, the value of the electrical property based on the stored data is compared with the values of the electrical property based on the “1” data and the “0” data, to determine the value of the stored data.
As described above, the reading operation of the self-reference type is basically destructive reading. As a consequence, where the stored data is “1”, for example, it is necessary to rewrite “1” data after determining the value of the stored data. In addition, it requires complicated steps to complete reading, which reduces the reading operation speed and hinders realization of a high speed memory. Furthermore, it is accompanied by two writing actions, which increase the power consumption in reading.
Accordingly, it is preferable to provide an MRAM, which can perform reading with a small number of errors, by a reduced number of steps as compared to the reading operation of the self-reference type, and by nondestructive reading.